At present, a thermal printer is applied more and more widely. A printing mechanism of the thermal printer includes a thermal printing head and a printing platen, the thermal printing head and the printing platen are tangentially arranged, printing supplies penetrate through a part between the thermal printing head and the printing platen, multiple heating units arranged in a line at an equal interval along a width direction of the printing head are arranged on the printing head, and when the printer executes a printing task, the heating units are controlled to emit heat, thereby produce energy (called printing energy for short), and the printing energy is transferred to the printing supplies contacting with the thermal printing head, wherein the printing supplies in a direct thermal printer refers to a thermal printing medium, and a thermal layer on a surface of the thermal printing medium is chemically changed by the printing energy, thereby developing a colour and forming printing dots corresponding to the heating units one to one; and in a thermal transfer printer, the printing supplies include a common printing medium and a thermal transfer ribbon, and a pigment substance on the thermal transfer ribbon is heated by the printing energy, and melted and transferred onto a surface of the common printing medium to form the printing dots corresponding to the heating units one to one.
When the parallelism of the printing head and the printing platen installed in the printer is inconsistent under the influence of an assembly error or a machining error, pressure exerted on the printing platen by the printing head is non-uniform, so that the printing energy transferred to the printing supplies is inconsistent when the heating units emitting heat, a printing density of each printing dot on the printing medium is non-uniform, and an abnormal printing effect is caused. FIG. 1 shows a printed image with an abnormal printing effect caused by non-uniform pressure exerted on the printing platen by the printing head, and in the image, the printing energy transferred to the printing supplies is non-uniform because the pressure exerted on the printing platen by the printing head is non-uniform, and “white spots” as shown in FIG. 1 are formed on parts with insufficient printing energy.
Chinese patent with the application number of CN200310120787.7 discloses a method for compensating printing energy, and according to the method, the printing energy output by the printing head in a printing dot line with a requirement on printing energy compensation is compensated by regulating a duration of a valid strobe signal applied to the printing head; however, when the printing energy is compensated by the method, the printing energy output by all the heating units of the printing head required to emit heat in printing a dot line may be compensated, and the printing density of the printing dot line of the image is improved overall; therefore, by the method, the problem of non-uniform printing density of each printing dot caused by non-uniform pressure exerted on the printing platen by the printing head cannot be solved.
For the problem that the non-uniform printing density of each printing dot caused by non-uniform pressure exerted on the printing platen by the printing head cannot be effectively avoided by printing energy compensation in a related art, there is yet no effective solution.